Thermal barrier coating systems and materials

ABSTRACT

A new family of ceramic materials is identified having particular utility as thermal insulating or thermal barrier coatings on metallic substrates. The ceramic materials have a pyrochlore structure and are typified by the composition A 2  B 2  O 7  where A and B are various ions and O is oxygen. A may have a positive charge of 3 +   or 2 +   and B may have a positive charge of 4 +   or 5 30 . These materials are characterized by having chemical stability, thermal stability and thermal insulating properties superior to those of currently used thermal barrier ceramics. An example pyrochlore material is lanthanum zirconate.

FIELD OF THE INVENTION

This invention relates to a class of ceramic materials for thermalbarrier coatings, to thermal barrier coatings made of such materials,and to metallic parts having such thermal barrier coatings. The thermalbarrier coatings have particular utility in gas turbine engines. Theceramic materials are a family of ceramics having a pyrochlorestructure.

BACKGROUND OF THE INVENTION

Gas turbine engines are well developed mechanisms for convertingchemical potential energy, in the form of fuel, to thermal energy andthen to mechanical energy for use in propelling aircraft, generatingelectric power, pumping fluids etc. At this time the major availableavenue for improved efficiency of gas turbine engines appears to be theuse of higher operating temperatures. However the metallic materialsused in gas turbine engines are currently very near their upper limitsof thermal stability. In the hottest portion of modern gas turbineengines, metallic materials are used at gas temperatures above theirmelting points. They survive because they are air cooled. But excessiveair cooling reduces engine efficiency.

Accordingly, there has been extensive development of thermal barriercoatings for use with cooled gas turbine aircraft hardware. By using athermal barrier coating, the amount of cooling air required can besubstantially reduced.

Such coatings are invariably based on ceramics; mullite, alumina, etc.have been proposed but zirconia is the current material of choice.Zirconia must be modified with a stabilizer to preserve its cubiccrystal structure at elevated temperatures, typical stabilizers includeyttria, calcia, ceria and magnesia.

Generally speaking, metallic materials have coefficients of thermalexpansion which exceed those of ceramic materials, consequently one ofthe problems that must be addressed in the development of successfulthermal barrier coatings is to match the coefficient of thermalexpansion of the ceramic material to the metallic substrate so that uponheating, when the substrate expands, the ceramic coating material doesnot crack. Zirconia has a high coefficient of thermal expansion and thisis a primary reason for the success of zirconia as a thermal barriermaterial on metallic substrates.

Thermal barrier coatings have been deposited by several techniquesincluding thermal spraying (plasma, flame and HVOF), sputtering andelectron beam physical vapor deposition (EBPVD). Of these techniques,electron beam physical vapor deposition is currently a preferredtechnique for demanding applications because it produces a uniquecoating structure. Electron beam physical vapor deposited ceramicmaterials, when applied according to certain parameters, have a columnargrain microstructure consisting of small columns separated by gaps whichextend into the coating. These gaps allow substantial substrateexpansion without coating cracking and/or spalling see U.S. Pat. No.4,321,311. According to U.S. Pat. No. 5,073,433 a similar structure(comprising segmentation cracks), although on a larger scale, can beobtained by plasma spray techniques.

Despite the success with the current use of electron beam physical vapordeposited zirconia base coatings there is a continuing desire forimproved coatings which exhibit superior thermal insulationcapabilities, especially improved in insulation capabilities whennormalized for coating density. Weight is always a critical factor whendesigning gas turbine engines, particularly in rotating parts. Ceramicsthermal barrier coatings are not load supporting materials, consequentlythey add weight without increasing strength. There is a strong desirefor a ceramic thermal barrier material which adds the minimum weightwhile providing the maximum thermal insulation capability. In additionthere are obviously the normal desires for long life, stability, economyetc.

Although this coating was developed for application in gas turbineengines, the invention clearly has utility in other applications wherehigh temperatures are encountered such as furnaces.

DESCRIPTION OF THE DRAWINGS

FIG. 1a depicts the crystal structure of lanthanum zirconate, apyrochlore.

FIG. 1b depicts the crystal structure of zirconia, a fluorite structure.

FIG. 2 shows the relationship between the A and B constituent ionicsizes necessary to produce a pyrochlore structure.

FIG. 3a depicts a ceramic coating directly on a metallic substrate.

FIG. 3b depicts a ceramic coating on a metallic substrate with anintermediate bond coat.

FIG. 3c depicts an expanded view of the interface between the bond coatand the ceramic layer in FIG. 3b.

FIG. 4 shows the ZrO₂ -La₂ O₃ phase diagram.

FIG. 5 shows the thermal conductivity of several ceramic materials.

FIG. 6 shows the coefficient of thermal expansion for several ceramicmaterials.

FIG. 7 shows an X-ray diffraction scan.

SUMMARY OF THE INVENTION

The essence of the present invention arises from the discovery that aclass of ceramic materials has great utility as thermal barrier coatingson metallic substrates. These materials have a pyrochlore crystalstructure.

The term pyrochlore is used to identify an ore of tantalum found inCanada. The term more generally describes a ceramic structure of thecomposition A₂ B₂ O₇ where A can have valance of 3⁺ or 2⁺ and B can havea valance of 4⁺ or 5⁺ and wherein the sum of the A and B valences is 7.The oxygen can be replaced by in part by sulfur or fluorine. Typicalpyrochlores which we believe to have potential as thermal barriercoatings are those in which A is selected from the group consisting oflanthanum, gadolinium and yttrium and mixtures thereof and B is selectedfrom the group consisting of zirconium, hafnium and titanium andmixtures thereof. Many other pyrochlores exist which also have potentialas thermal barrier materials. See "Oxide Pyrochlores--A Review" by M. A.Subramanian et al, Progress in Solid State Chemistry, vol 15, pp 55-143,1983 (incorporated herein by reference) for a full description ofpyrochlores.

We have found that on a density adjusted basis pyrochlores which we haveinvestigated have thermal insulating properties which exceed those ofthe more commonly used zirconia based thermal barrier materials.Additionally many of the pyrochlore materials have a phase relationshipin which the pyrochlore structure is phase stable up to the meltingpoint. Consequently stabilizing additions are not required. Most of thepyrochlores which we have investigated have melting points of more than3000° F. (1650° C.) and generally more than 4000° F. (2200° C.).Additionally these materials adhere to alumina. These are all propertieswhich are useful in thermal barrier coatings.

The invention coating materials and coatings will usually be used toprotect a superalloy substrate from excess temperatures. Superalloys aremetals, usually based on iron, nickel or cobalt and containing chromiumand aluminum and usually titanium and refractory metals, and havinguseful properties above 1200° F. (650° C.). Other substrates, includingsteels, copper alloys and titanium alloys may be protected. Table Idescribes exemplary substrate materials.

                                      TABLE I                                     __________________________________________________________________________    (wt % Exemplary Supperalloy Compositions)                                           Cr Co W  Cb                                                                              Ti                                                                              Al                                                                              B  Hf                                                                              C Ni                                                                              Ta Mo                                                                              Zr                                                                              Re                                       __________________________________________________________________________    PWA1422                                                                             9  10 12 1 2 5 .015                                                                             1.6                                                                             .14                                                                             Bal                                                                             -- --                                                                              --                                                                              --                                         PWA1426 6.4 12.6 6.4 -- --  5.9 0.012 1.5 -- Bal 3.0 1.7 .08 .3                                                   PWA1480 10 5 4 -- 1.5 5 --  -- --                                            Bal 12 -- -- --                            IN 792 12 9 3.8 -- 4.1 3.5 .015 0.5 .12 Bal 3.9 1.9 .12 --                  __________________________________________________________________________

As in other ceramic thermal barrier coatings, adherence of thepyrochlore ceramic to the alloy substrate is critical.

It is known from prior zirconia thermal barrier coatings that a metallicbond coat (sometimes described as an overlay coating) such as a MCrAlYis a superior bond coat for oxide ceramic coatings. It is also knownthat aluminide coatings are useful bond coats through generally not asdurable as MCrAlY bond coats. The common feature of overlay coatings andaluminide coatings is that they both form adherent alumina surface filmsor scales.

A broad composition range for MCrAlY materials is 10-25% Cr, 5-15 Al,0.1-1.0 Y balance selected from Fe, Ni, and Co and mixtures of Ni andCo. Additions of up to 5% each of Hf, Ta or Re, up to 1% of Si and up to3% each of Os, Pt, Pd, or Rh may also be made. Table II describesexemplary MCrAlYs that can be applied by thermal spray processes, byEBPVD processes, and by electroplating.

                  TABLE II                                                        ______________________________________                                        (wt % Exemplary MCrAlY Compositions)                                                     Ni    Co      Cr   Al     Y   Hf    Si                             ______________________________________                                        NiCrAlY    Bal   --      19.5 12.5   .45 --    --                               CoCrAly -- Bal 18 11 .45 -- --                                                NiCoCrAlY Bal 23 18 12.5 .3 -- --                                             NiCoCrAlY Bal 22 17 12.5 .6 .25 .4                                          ______________________________________                                    

An alternate bond coat is a diffusion aluminide formed by diffusingaluminum into the substrate surface. Diffusion aluminides are well knownand may be applied using a mixture (termed a pack) containing analuminum source, such as an aluminum alloy or compound, an activator(usually a halide compound such as NaF) and an inert material such asalumina. The part to be coated is buried in the pack and heated to1500-2000° F. while a carrier gas, such as hydrogen, is flowed throughthe pack out of pack processes wherein the part is not buried in thepack are also known. The incorporation of precious metals such as Pt,Rh, Pd and Os into aluminide coatings is known. See U.S. Pat. No.5,514,482 for a description of aluminide coating processes.

Combinations of overlay and aluminide coatings are also possible. SeeU.S. Pat. No. 4,897,315 for a description of a system having an innerMCrAlY overlay coating and an outer aluminide coating. See U.S. Pat. No.4,005,989 for a description of the reverse combination, an inneraluminide coating and an outer overlay coating.

The common feature of these bond coats and bond coat combinations isthat they form an adherent layer of alumina on their outer surface. Theinvention thermal barrier coating has limited solubility in alumina butbonds firmly to the alumina.

In certain cases, superalloys may form sufficiently perfect and adherentalumina layers that ceramics may adhere without a separate bond coat.See U.S. Pat. Nos. 5,262,245, 4,895,201, 5,034,284, 5,346,563, and5,538,796.

To date all successful applications of ceramic coatings to superalloyhas included oxide layer (usually alumina, rarely silica) between thebond coat (or substrate) and the ceramic coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The pyrochlore structure is a complex structure which can be describedin a variety of ways, as a fluorite structure derivative, or as anetwork of octahedral linked corner to corner with cations filling theinterstices.

FIG. 1a is a depiction of a cubic pyrochlore crystal structure.Regardless of structure description the pyrochlore structure has achemical composition of A₂ B₂ O₇ or occasionally A₂ B₂ O₆ or AB₂ O₆ withthe latter two being referred to as defect pyrochlores. FIG. 1aillustrates lanthanum zirconate having an A₂ B₂ O₇ chemistry. FIG. 1bshows a cubic fluorite structure, the structure of stabilized zirconia.A comparison of FIGS. 1a and 1b shows both similarities and differencesbetween the two structures. Both FIGS. 1a and 1b are views down the<100> crystal axis. Visually the pyrochlore structure appears to be lessregular than the fluorite structure.

The A and B ions can have different valances as long as the sum of the Aand B valences adds up to 7, in the case of the A₂, B₂ O₇ or 6 in thecase of A₂ B₂ O₆ structures.

The pyrochlore structure forms only for certain relationships of A and Bionic radii. FIG. 2 illustrates this relationship indicating the generalcombination of A and B ionic radii which produce cubic pyrochlores. Wehave found that the boundaries of this diagram are somewhat uncertainand we believe, based on our investigations, that lanthanum titanate[La₂ Ti₂ O₇ ] has a stable cubic pyrochlore structure.

Noncubic pyrochlores are known but for the purpose of this invention weprefer to use ceramics which have a cubic pyrochlore structure.

As shown in FIG. 2, the formation of the desired cubic pyrochlorecrystal structure is controlled by the relative ionic radii of the A andB constituents. It is possible to use a mixture of elements for the Aand/or B constituents to provide an average ionic radius which willcause formation of a cubic pyrochlore structure. By way of example, fromFIG. 2 it can be seen that both Gd₂ Ti₂ O₇ and Y₂ Zr₂ O₇ will have cubicpyrochlore structures. As a general rule, compounds of the formula(Gd_(x) Y_(y))(Ti_(a) Zr_(b))O₇, where x+y=2 and a+b=2, will also have acubic pyrochlore structure.

Further, a compound such as ln₂ Zr₂ O₇ which is not cubic could likelybe rendered cubic through partial substitution of (for example) Nd forln and/or Ti for Zr, to bring the average A and B ionic radii into thecubic pyrochlore region shown in FIG. 2.

We have worked with the type of pyrochlores denoted by the A₂ B₂ O₇formula and of these we prefer to use gadolinium, lanthanum or yttriumfor the A ionic species and hafnium, titanium or zirconium for the Bionic species. Lanthanum zirconate seems particularly useful becauselanthanum and zirconium have similar vapor pressures, making vapordeposition more readily possible. We have used only the materials of theA₂ B₂ O₇ structure and have not attempted the use of the knownpyrochlores which have either fluorine or sulfur to substitute a portionof the oxygen but we do not believe that there is any reason to excludethe sulfur and fluorine substituted compositions from the presentinvention. We have also not experimentally evaluated the A₂ B₂ O₆ andAB₂ O₆ structures but believe that they also may have utility in thermalbarrier coatings.

Ti, Zr and Hf all display complete solid solubility in each other and webelieve that any combination of Ti+Zr+Hf can be used as the B ionicspecies. Similarly, Gd, La and Y have substantial solid solubilities(Gd-La has complete solubility). Any combination of Gd+La+Y which doesnot form a second phase can be used as the A ionic species. These alloysof the A and B species must satisfy the criteria of FIG. 2 and possess apyrochlore structure.

The low thermal conductivity of oxide pyrochlore compounds can berationalized by consideration of crystallographic and chemical effectson thermal conductivity. The thermal conductivity of dielectric solidsat elevated temperature is determined by phonon scattering by crystalimperfections and other phonons. Oxide pyrochlore compounds exhibit manyof the features associated with low thermal conductivity materials. Thepyrochlore crystal structure has a high intrinsic defect concentration.It has been experimentally established that as the difference in atomicmass between constituents in a compound increases, the thermalconductivity of that compound tends to decrease. Although the pyrochloreand fluorite structure are closely related, substitution of a highconcentration of high atomic mass atoms (lanthanum, gadolinium andyttrium) into the fluorite structure provides a means to lower thermalconductivity that does not readily exist with stabilized zirconiacompounds. It should be noted that, for thermal barrier applications,the benefits obtained from the reduction in thermal conductivityresulting from the use of high atomic mass elements must out weigh thedebit incurred from higher density.

Reduction in thermal conductivity has also been associated withincreasing complexity of crystallographic structure. As shown in FIG.1a, the pyrochlore structure exhibits a greater degree of complexitythan the fluorite structure shown in FIG. 1b. The cubic pyrochlorestructure is similar to the cubic fluorite structure but with a largenumber of the oxygen atoms displaced (and one in eight missing).

Thermal barrier coatings are typically applied by thermal sprayprocesses, such as plasma spray, in air (APS) or in low pressure (LPPS)by high velocity oxygen fuel processes (HVOF) or by detonation guns (DGun). Electron beam physical vapor deposition (EBPVD) and sputtering areother techniques. Electron beam physical vapor deposition is a favoredprocess. Depending upon the application and circumstances, each processhas particular advantages. All of these processes can be readily used toapply oxide pyrochlore thermal barrier coatings. As previouslydiscussed, the EBPVD process offers advantages since it develops astructure suited for extreme temperature applications and is thereforemost suitable for coating hot section turbine components. Thermal sprayprocessing offers advantages in coating large components of complexshape and would be most suitable in coating components such ascombustors.

FIGS. 3a, b and c illustrates variants of the thermal barrier coatingsof the present invention. FIG. 3a depicts a coated article whichcomprises a superalloy substrate 10 having a pyrochlore top coat 20 onits outer surface 21. In gas turbine applications the backside 11 of thesuperalloy substrate 10 will be cooled by cooling air (not shown) andthe outside front surface, or free space, 21 of the pyrochlore will beexposed to elevated temperatures. There may also be holes between theouter surface and the backside permitting cooling air to flow from thebackside to the outer surface. Angled and shaped cooling holes incombination with flowing hot gases on the outer surface can produce filmcooling in which a layer of cool air separates the outside surface fromthe hot gases to further reduce heat flow. Heat will flow from the frontsurface 21 to the cooled surface 11 and the quantity of the heat flowwill be substantially reduced by the pyrochlore layer. As previouslymentioned the pyrochlore may be applied by various methods and themacrostructure of the pyrochlore layer will be largely a function of thedeposition process. The most basic invention embodiment is a pyrochlorelayer adhered to a substrate which reduces heat flow in the presence ofa thermal gradient.

FIG. 3b illustrates a preferred construction which employs a bond coat15 between the substrate 10 and the pyrochlore 20. The bond coat 15improves adherence and provides oxidation protection for the substrate.FIG. 3c is an expanded view of the interlayer 16 between the bond coat15 and the pyrochlore layer 20. An oxide layer 22, principally ofalumina, exists at this interlayer and is believed to be principallyresponsible for the adherence of the pyrochlore.

It is known to augment the naturally occurring alumina layer on the bondcoat by sputtering alumina onto the bond coat, in the case of zirconiathermal barrier coatings, and the use of a separately applied aluminalayer (rather than a thermally grown oxide layer) is also an embodimentof this invention.

In a further embodiment another ceramic layer may be applied to the freesurface of the pyrochlore. This added layer can be selected to reduceoxygen diffusion, to provide erosion and abrasion resistance, or toprovide a desired thermal emissivity characteristic, or some combinationof these characteristics.

EXAMPLE I

The use of the La₂ Zr₂ O₇ (lanthanum zirconate) pyrochlore oxidecompound as a EBPVD applied thermal barrier coating will be illustrated.The advantageous properties of the La₂ Zr₂ O₇ pyrochlore oxide relativeto stabilized zirconia for thermal barrier coating include thermalconductivity, thermal expansion, density, and phase stability. FIG. 4shows the La₂ O₃ -ZrO₂ phase diagram with the pyrochlore phase fieldlabeled P. It can be seen that the pyrochlore structure (at about 35 mol% La₂ O₃) is stable up to the melting point at about 2300° C. (4172°F.).

FIG. 5 shows thermal conductivity of La₂ Zr₂ O₇ compared to the thermalconductivity of cubic zirconia as a function of temperature. At typicalthermal barrier coating use temperatures, the pyrochlore compoundexhibits a thermal conductivity which is about 50% that of stabilizedzirconia. The density of the La₂ Zr₂ O₇ pyrochlore compound isapproximately the same as stabilized zirconia (approximately 6gr/cm³) soon a weight corrected basis, the thermal conductivity benefit is alsoabout 50%.

To illustrate the benefit, the 50 percent reduction thermal conductivityallows the coating thickness to be reduced by 50 percent for the samedegree of thermal protection. Decreasing the coating mass by 50 percenton a typical turbine blade will lower the blade pull at the blade rootby approximately 1,500 pounds (680 Kg), at typical operating conditions,which results in a significant increase in blade life and permits areduction in mass of the disk to which the blades are attached. If thethermal barrier coating were held at the same thickness, and the coolingair flow held constant, the substrate temperature would be reduced byabout 100° F. (55° C.), giving increased substrate creep life. Keepingthe coating thickness constant and reducing the airflow would increaseengine efficiency.

FIG. 6 shows the mean coefficient of thermal expansion of La₂ Zr₂ O₇compared to that of cubic stabilized zirconia as a function oftemperature. The thermal expansion of La₂ Zr₂ O₇ thermal barrier coatingcan be seen to be similar to that of the cubic zirconia thermal barriercoating. This means that La₂ Zr₂ O₇ will behave similarly to zirconiaduring thermal cycling.

EXAMPLE II

Lanthanum zirconate was applied to a substrate by electron beam vapordeposition (EBPVD), in a controlled atmosphere chamber. The coating wasapplied to a single crystal substrate (of the nominal composition of PWA1480 (see Table II)). The coating process was carried out in a vacuum of3.2×10⁻⁴ Torr with an oxygen flow rate of 50 sccm. Oxygen was added toensure pyrochlore oxygen stoichiometry, see U.S. Pat. No. 5,087,477. Thesubstrate temperature was 1840° F. during deposition with a substrate tosource distance of 5.25 inches. The source pyrochlore ceramic wasevaporated with an electron beam run at 0.8 A and 10,000 V. The sourceoxide was La₂ Zr₂ O₇ powder. The coating exhibited the favorablecolumnar grain structure typical of electron beam physical vapordeposited cubic zirconia thermal barrier coatings that provides strainrelief and improved durability over plasma sprayed coatings.

FIG. 7 shows an X-ray diffraction scan obtained from the surface of thecoating. The diffraction peaks have been indexed to the pyrochlorecrystal structure which demonstrates that the pyrochlore structure wasformed in the deposited thermal barrier coating.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be understood by those skilled inthe art that various changes, omissions and additions in form and detailthereof may be made without departing from the spirit and scope of theclaimed invention.

I claim:
 1. A metallic article comprising a metallic substrate, saidsubstrate having a thermally-insulating ceramic coating on its surfacewherein said ceramic coating has a cubic pyrochlore structure.
 2. Anarticle as in claim 1 wherein said metallic substrate is selected fromthe group consisting of steels, superalloys, titanium alloys and copperalloys.
 3. An article as in claim 1 wherein said pyrochlore has theformula A₂ B₂ O₇ and the A and B species ionic radii fall within thecubic field in FIG.
 2. 4. An article as in claim 3 wherein the Bconstituent comprises Hf, Ti, Zr, and single phase mixtures thereof. 5.An article as in claim 3 wherein the A constituent comprises La, Gd, Y,and single phase mixtures thereof.
 6. An article as in claim 1, thepyrochlore coating having a free surface and wherein said coated articleis adopted to be used in environments where the free surface of thepyrochlore coating will be heated and the free surface of the substratewill be cooled, whereby the pyrochlore coating will reduce heat flow. 7.A metallic article as in claim 1 wherein said metallic article has anoxide scale on its outer surface, said oxide consisting essentially ofalumina, and wherein said cubic pyrochlore ceramic coating is bonded tosaid oxide scale.
 8. An article as in claim 7 wherein said metallicsubstrate is selected from the group consisting of steels, superalloys,titanium alloys and copper alloys.
 9. An article as in claim 7 whereinsaid pyrochlore has the formula A₂ B₂ O₇ and the A and B species ionicradii fall within the cubic field in FIG.
 2. 10. An article as in claim9 wherein the B constituent comprises Hf, Ti, Zr, and single phasemixtures thereof.
 11. An article as in claim 9 wherein the A constituentcomprises La, Gd, Y, and single phase mixtures thereof.
 12. An articleas in claim 7, the pyrochlore coating having a free surface and whereinsaid coated article is adopted to be used in environments where the freesurface of the pyrochlore coating will be heated and the free surface ofthe substrate will be cooled, whereby the pyrochlore coating will reduceheat flow.
 13. An article as in claim 7, wherein said pyrochlore has acolumnar microstructure.
 14. An article as in claim 1, wherein saidpyrochlore has a columnar microstructure.
 15. A metallic articlecomprising a metallic substrate, said substrate having an aluminaforming coating on its surface and a thermally insulating pyrochlorecoating bonded to said alumina forming coating.
 16. An article as inclaim 15 wherein said substrate coating comprises an alumina formingmetallic overlay coating.
 17. An article as in claim 15 wherein saidsubstrate coating comprises a diffusion aluminide coating.
 18. Anarticle as in claim 15, the pyrochlore coating having a free surface andwherein said coated article is adopted to be used in environments wherethe free surface of the pyrochlore coating will be heated and the freesurface of the substrate will be cooled, whereby the pyrochlore coatingwill reduce heat flow.
 19. An article as in claim 15 wherein saidpyrochlore has a columnar microstructure.
 20. An article as in claim 15wherein the pyrochlore has the formula A₂ B₂ O₇ and the A and B speciesionic radii fall within the cubic field of FIG.
 2. 21. An article as inclaim 20 wherein the B constituent comprises Hf, Ti, Zr, and singlephase alloys thereof.
 22. An article as in claim 20 wherein the Aconstituent comprises La, Gd, Y, and single phase alloys thereof.